In projectors which use an image-forming element such as a liquid crystal panel, it is important to use light from a light source efficiently, reduce unevenness of illuminance on a screen, and display images having a high contrast. Many methods to improve them have been proposed in the past.
For example, an optical integrator has been known which is constituted by two lens array, each having lens cells arranged two-dimensionally. In this optical integrator, a first lens array divides a luminous flux from a light source into a plurality of luminous fluxes, and a second lens array performs an image formation on the display area of the liquid crystal panel with enlargement and superposition of the plurality of luminous fluxes, thereby obtaining illumination light with a high evenness (see Japanese Patent Laid-Open No. H03-111806).
Further, a method has been proposed for achieving evenness of illumination by a substantially parallel light beam transmission in a first direction and an optical integration in a second direction orthogonal to the first direction (see Japanese Patent Laid-Open No. 2004-245977 and No. 2004-271887). The first direction is a direction in which deterioration of contrast occurs due to an incident angle dependency of color separating or combining elements used in projectors, the elements including deflection elements such as a dichroic mirror, polarization beam splitting elements such as a polarization beam splitter, and that of liquid crystal panels.
On the other hand, projectors require not only displaying of bright images but also preventing of observer's feeling of glare by not displaying too-bright images in a bright environment. In other words, the projectors require a function to suppress the brightness of displayed images in accordance with the brightness of the use environment while maintaining a high contrast characteristic or improving those contrast characteristic.
For achieving this required function, a method has been proposed in Japanese Patent Laid-Open No. 2004-151674 which suppresses the brightness of displayed images by using an optical integrator that includes two lens array, each having lens cells arranged two-dimensionally, and means for variably shielding the illumination luminous flux while improving the contrast characteristic.
However, a problem occurs when the means for variably shielding the illumination luminous flux, which has been proposed in Japanese Patent Laid-Open No. 2004-151674, is simply used in the illumination optical system that controls separately in the two-dimensional axis directions the optical integrator and the light intensity distribution converting means for converting the light intensity distribution, which has been described in Japanese Patent Laid-Open No. 2004-245977 and No. 2004-271887.
Specifically, since the illumination optical system is a system that adopts different illumination light transmission methods in the two directions, there is a possibility that a balance of even illumination of liquid crystal panels may disrupt and the evenness of the displayed image may be not maintained.